Near-infrared (NIR) spectroscopy, a noninvasive optical method for
monitoring hemodynamics, was applied to record the evoked auditory response of
the human brain. A newly developed system consists of a halogen light source, an
NIR spectroscope, and optical fiber bundles for the light and signal probes. The
system detects the amount of oxygenated and deoxygenated hemoglobin in the area
of the local brain located beneath the probes. In this study,
magnetoencephalography (MEG) and NIR spectroscopy were used to observe the blood
component change near the auditory cortex. The sound stimulus of a 1-kHz sine
wave at 70 dB SPL was used to elicit evoked responses in both experiments. The
active region of the auditory cortex was determined by the MEG, and the same
area was examined by the NIR system. The results from NIR experiments showed
significant increases of the total hemoglobin and deoxygenated hemoglobin,
indicating both blood flow and oxygen consumption at the cortical area increased
in response to sound stimuli. The NIR recordings at several different depths
revealed that the increases of local blood flow and oxygen consumption were
observed only at depths ranging between 20 and 30 mm from the scalp.